[6.x] Implement integration test and in-memory DB (#5169)

* Use in-memory DB for testing

* Extend from PHPUnit test case for unit tests

phpunit.xml

@@ -28,6 +28,8 @@
         <server name="APP_ENV" value="testing"/>
         <server name="BCRYPT_ROUNDS" value="4"/>
         <server name="CACHE_DRIVER" value="array"/>
+        <server name="DB_CONNECTION" value="sqlite"/>
+        <server name="DB_DATABASE" value=":memory:"/>
         <server name="MAIL_DRIVER" value="array"/>
         <server name="QUEUE_CONNECTION" value="sync"/>
         <server name="SESSION_DRIVER" value="array"/>

tests/Unit/ExampleTest.php

@@ -2,8 +2,7 @@
 
 namespace Tests\Unit;
 
-use Illuminate\Foundation\Testing\RefreshDatabase;
-use Tests\TestCase;
+use PHPUnit\Framework\TestCase;
 
 class ExampleTest extends TestCase
 {